Phytoplankton may stimulate uptake of CO2

04/13/04

CORVALLIS - New research has revealed that phytoplankton may be one of the main historic controls on global warming, and that fertilizing the oceans with iron results in increased phytoplankton productivity – a hypothetical way to offset the effects of global warming.

Through photosynthesis, these tiny, free-floating aquatic plants can convert carbon dioxide to organic carbon, and there appears to be a prehistoric relationship between iron in the ocean and atmospheric levels of carbon dioxide.

Burke Hales, an assistant professor in the College of Oceanic and Atmospheric Sciences at Oregon State University, is one of a number of scientists who collaborated on a new study which involved field research in the ocean near Antartica and will be published Friday in the journal Science.

He described the research as "tremendously successful" because it very clearly shows an induced biological response in the oceans to fertilization with iron.

"During the glacial periods, atmospheric carbon dioxide, or CO2 levels decrease substantially, while during interglacial periods, such as we are now in, those levels increase," said Hales. "There is also a striking inverse relationship between implied, historical iron fluxes to the ocean and atmospheric CO2 concentrations. These relationships suggest some sort of feedback system between iron and CO2 levels during glacial periods that keep the temperature low."

The carbon cycle is a complicated system of causes and effects that are not completely understood, but researchers have long suspected that the oceans are the main regulator of the Earth's atmosphere, said Hales. For example, during the ice ages more of the Earth's water is locked up in glaciers, creating arid, windy conditions and a lot of dust. This iron-rich dust is blown out to sea, stimulating productivity of phytoplankton throughout the world's oceans and reducing CO2 levels.

"In order for the phytoplankton to be a long-term sink for carbon, they somehow have to get deposited in the deep ocean, and that doesn't always happen," said Hales. "If the phytoplankton are just eaten at the surface, or don't sink to any great depth then the carbon is eventually released back into the atmosphere."

Another complication in phytoplankton production is the availability of silicate, which is potentially a limiting factor in the growth of certain types of phytoplankton. Diatoms are a large type of phytoplankton that have siliceous shells, and because of their relative bulkiness have a higher probability of sinking into the deep ocean for longer periods of time. So it seems logical that iron-fertilized, low silicate waters might not be as efficient carbon sinks as iron-fertilized high silica waters, but the results of this study disproved that idea for the first time.

"This was the first experiment of this nature in low silicate waters where it didn't seem as though there would be enough silica for the diatoms to grow," Hales said. "However, our results showed an enhanced uptake of atmospheric CO2 in the fertilized region despite the low availability of silicate."

Since humans starting burning fossils fuels, CO2 levels have skyrocketed and there has been increasing concern over the role that has played in global warming.

"The difference between the amount of CO2 in the atmosphere today and during pre-industrial times is about the same as the difference between interglacial and glacial periods," said Hales. "There is definitely a correlation between the amount of CO2 in the atmosphere and global warming, but the relationship is hard to define."

Hales' role in the study involved developing apparatus to sample the ocean water and measure the concentrations of various chemicals, such as nitrate, phosphate, silicates and dissolved CO2 in order to determine the impact on levels of atmospheric carbon dioxide.

"We needed very high spatial resolution measurements of chemicals in the fertilized regions, so the technology we used allowed us to take fairly continuous samples," said Hales. "The sampler was something like a little underwater airplane that continuously pumped water up to the ship while soaring up and down in the water as we towed it."

Although Hales is excited about the scientific implications of the research, such as the insight it provides into the relationship between the glacial and interglacial cycles with the CO2 record, he is reluctant to make any claims that fertilizing the ocean with iron would realistically help control global warming.

"There are so many repercussions that we can't foresee," said Hales. "This is a very expensive and uncertain way of going after an issue that is not fully understood. For example, in the process of gathering up iron and steaming out to sea, you would burn up more fossil fuel than you would compensate for in the result. Besides that, there's also the issue of shifting an ecosystem structure that the food web is based upon by adding iron. We really have no idea what sort of positive or negative effects that would have."

Another huge unknown in the experiment are the effects of time, cautioned Hales. The time scale of the experiment, 42 days, is not at all comparable to the time scale of the glacial/interglacial cycle, which is thousands of years.

"We weren't even out there long enough to observe the season to season changes, so we don't know if the carbon was really being exported to the deep oceans or not," Hales said. "A longer term study would be necessary to draw more concrete conclusions."

Source: Eurekalert & others

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